No one uses Flexbox in production. You're better off learning real CSS if you're just getting started with web development.
Flexbox is real CSS, it's part of the CSS3 spec: http://www.w3.org/TR/2015/WD-css-flexbox-1-20150514/
However, this webapp is used to generate websites that may be visited by anybody, really, so we have to support IE9 and we don't use flex box for them. It all depends on your customers / visitors base.